In accordance with the recent marked advance of the LSI technology, the degree of integration of LSI is greatly increased with an attendant increase of heat release per package. The heat sink ability of substrate materials is of greater importance. Thus aluminum nitride substrates having good heat dissipation now draw attention as a substitute for the conventional alumina substrates.
In general, aluminum nitride substrates are prepared by molding aluminum nitride powder under pressure and firing the compacts. Since aluminum nitride itself is less susceptible to sintering, sintering aids are often added to the aluminum nitride powder prior to sintering. It is well known to use calcium oxide and yttrium oxide as the sintering aids as disclosed in Japanese Patent Publication Nos. 49510/1983 and 9475/1981, respectively.
These sintering aids, however, have both advantages and disadvantages and are not fully satisfactory as sintering aids for aluminum nitride. More particularly, the addition of yttrium oxide has the advantage that the sintered aluminum nitride shows high heat transfer, but the disadvantages that its source material is expensive, the sintering temperature must be as high as 1,800.degree. C. or higher, and grain boundary precipitation occurs during sintering so that the resulting sintered bodies have poor appearance. In turn, calcium oxide is cost effective because its source material is inexpensive, but the sintered bodies show low heat transfer.